A Micro-Electro-Mechanical Systems (MEMS) mirror has been used as a micro-sized optical scanner. An optical scanner is used in a laser printer or an image display apparatus, for example. When an optical seamier is used in an image display apparatus, a field of view of a display image depends on a deflection angle of the optical scanner. Accordingly, it is advantageous to use the optical scanner having a larger deflection angle to display a large and precise image. A method of driving an optical scanner at a resonance frequency of the optical scanner (resonance-drive) is effective to enlarge a deflection angle of the optical scanner. In addition, in order to achieve a larger deflection angle, it is advantageous to resonance-drive an optical scanner having a great Q value (typically, about 103) that indicates a resonance characteristic.
For resonance-driving of an optical scanner, a drive frequency for driving the optical scanner is set to be equal to a resonance frequency of the optical scanner. In particular, in order to resonate an optical scanner having a great Q value, a drive frequency is required to precisely coincide with a resonance frequency. However, the resonance frequency of the optical scanner is not always constant. In other words, the resonance frequency of the optical scanner varies by disturbances such as temperature change, secular change and the like. JP-A-2008-310301 describes a method of determining a resonance frequency of an optical scanner. The method includes up-down sweeping a drive frequency over a frequency range including a resonance frequency of the optical scanner. Then, based on at least two frequencies that cause a deflection angle to be maximum during the up-down sweep, the resonance frequency of the optical scanner is determined. Based on the determined resonance frequency, a drive signal is generated, so that a resonance driving can be performed.
A frequency characteristic of an optical scanner, i.e., a relation between a drive frequency and a deflection angle can be classified into two types, i.e., linear and non-linear frequency characteristic. In an optical scanner having a linear frequency characteristic, a drive frequency and a deflection angle correspond to each other one-to-one. In contrast, in an optical scanner having a non-linear frequency characteristic, a drive frequency and a deflection angle does not correspond to each other one-to-one.
In general, an optical scanner having a non-linear frequency characteristic can obtain a deflection angle larger than that of an optical scanner having a linear frequency characteristic at a predetermined drive voltage. In other words, in order to obtain a large deflection angle, the optical scanner having a non-linear frequency characteristic is suitable rather than the optical scanner having a linear frequency characteristic.
The technique described in JP-A-2008-310301 is proposed for an optical scanner having a linear frequency characteristic. Accordingly, that technique cannot be applied to an optical scanner having a non-linear frequency characteristic.